which is better for mx and why?

 

You'll find a large majority of MX'ers run link rear ends, they claim they handle the rougher tracks better which I believe is true. Here is some good reading on link setups vs. no link setups. Granted this is talking about dirk bikes but it's the same concept. In fact my CRF450 quad is running the CRF450 dirt bike linkage and it works awesome!